The invention relates generally to the field of analog-to-digital and current-to-frequency converting systems, and more specifically to readout systems for inertial instrumentation employing analog torquer current.
Inertial instruments such as free rotor gyros frequently use torquer coils which either interact with a permanent magnet or produce an attracting magnetic field to reposition the inertial element. The torquer coils are driven by a servo loop responsive to pickoffs which sense displacement. The inertial element is thus repeatedly returned to a null position. The output of the inertial instrument is ordinarily provided to a digital computer. In many cases, the output to the computer is provided by measuring the torquer current instead of the pickoff signals. In some systems, the torquer is energized by discrete uniform pulses of current. Thus the torquer current is already in a computer-compatible digital format since the pulses can be counted. However, in certain applications there are advantages to using torquer current in an analog amplitude format, which must then be converted to a digital format appropriate for the computer. The aim of providing a numerical output precisely proportional to input current amplitude is often hampered by an environment rife with stray magnetic fields and temperature and power supply variations.
One way of converting analog current or voltage to frequency is to use an integrating voltage-to-frequency converter. The input current is converted to a voltage drop across a fixed sense resistor. The voltage is continuously integrated and then rebalanced by precisely controlled current pulses. The frequency of the pulses is directly related to the voltage amplitude. However, the high parts count and cost associated with the integrating voltage-to-frequency converter plus its poor bias stability are serious disadvantages.